Loading maps for dissolved inorganic nitrogen, particulate nitrogen and

This section presents the results for the loading maps for DIN, PN and sediment (evaluated as TSS) in plume waters. A detailed description of the methodology and loading maps, their potential uses and limitations are presented in Appendix A1.10.

(a) Mapping annual DIN concentration in the GBR 2003-2015

The model-predicted DIN export to GBR lagoon is examined by its annual concentration (DIN, µg/L) over 13 years (Figure 5-10 and Figure 5-11). These maps provide an estimate of how far DIN can travel in GBR waters, and the areas more likely to have higher DIN concentration. The areas covered by model-predicted DIN vary over the 13 years analysed.

Overall, years with very large river discharge (> 65,000,000 ML), which occurred in 2008,

2009 and 2011, resulted in larger areas of DIN transport and exposure across the GBR. This is in agreement with previous observations about plumes in the GBR, where larger river discharge leads to larger extent of river plumes (e.g., Álvarez-Romero et al., 2013; Brodie et al., 2012; Devlin et al., 2012a, 2012b).

Although the number of contributing rivers did not change over the modelled years, the extent of land-sourced DIN influence was greater in the Wet Tropics and Burdekin NRM regions compared to the Mackay-Whitsunday and Fitzroy regions (note that river loads from the Cape York and Burnett-Mary NRM regions were not included in the model). Similar trends have been observed on the distribution and movement of other land-sourced pollutants and they were attributed to rainfall and land uses differences (Devlin et al., 2012b, 2013).

The highest model-predicted DIN concentration was observed in 2011, followed by 2012 and 2009, with maximums of 268 µg/L, 197 µg/L and 172 µg/L, respectively. The areas presenting higher DIN concentration were relatively constant over the years, with higher DIN values observed in the Wet Tropics and Mackay-Whitsunday NRM regions than the other regions. Even though the Burdekin River is responsible on average for > 36% of the DIN load accounted in the model, it is also responsible for 60% of the total discharge. The large Burdekin River discharge results in large plumes and consequently, relatively low DIN concentrations. The Wet Tropics NRM region is characterised by large areas of cropping lands (predominately sugarcane) in the coastal areas, and the Johnstone and Tully catchments together possess more than 80% of the total banana crop and 27% of the sugarcane plantation in this region (Waterhouse et al., 2014).

Figure 5-10: Dissolved inorganic nitrogen (DIN, µg/L) concentration in the GBR lagoon 2003 water year (c.a., 1 October to 30 September). ‘Max.’ stands for the highest DIN concentration. Named rivers are those with load data available and grey lines are the NRM region boundaries.

Figure 5-11: Dissolved inorganic nitrogen (DIN, µg/L) over the GBR lagoon 2004-2015 water years (c.a., 1 Octoberto 30 September). ‘Max.’ stands for the highest DIN concentration. Dots represent rivers with load data available and grey lines are the NRM region boundaries.

The preferential northward movement of the river plumes can result in increased model-predicted DIN concentration in areas that may not directly receive high DIN loads from their catchments. The contribution of DIN from rivers to the waters of each NRM region was determined by the amount of DIN exported from each river that reaches a particular NRM region, divided by the total amount of DIN in that region. Two periods were considered- the 2002-03 and 2010-11 water years (Figure 5-12), which represent the two extreme years of DIN loads discharged into the GBR lagoon over the 13 years analysed (3,029 tonnes and 29,958 tonnes, respectively). If a river presents a DIN contribution of 100% to a particular NRM region, which is the case for the Fitzroy River (Figure 5-12g and Figure 5-12h), this means that no other river included in the model contributes DIN to that NRM region.

Overall, rivers located within a marine NRM region were the main contributors to the presence of DIN in its waters, although this varied between years. For example, of the total DIN mass in the Burdekin NRM waters in 2002-03 (c.a., 589 tonnes), 76% came from the Burdekin River and 14% from the Haughton River, the two main rivers of the NRM region, and 6% from the Herbert River, 4% from the Proserpine River and <1% from the O'Connell River. In the 2010-11 season, the Burdekin River contributed 27% of the DIN in the Wet Tropics region due to the large Burdekin River discharge/plume (Figure 5-12b). Similar patterns occurred in the Mackay Whitsunday region when in 2010-11 16% of DIN in its waters was derived from the Fitzroy River. Conversely in 2002-03, the Fitzroy River had no DIN contribution to Mackay Whitsunday region.

These results indicate that the northward plume transport has the potential to increase the DIN load impact into zones outside of the NRM region. For example, the contribution of DIN loadings from the Burdekin River combined with the high DIN concentrations from the Wet Tropic rivers is in agreement with the supporting theories of land-based eutrophication as a potential trigger for crown-of-thorns starfish outbreaks (Brodie et al., 2005; Wooldridge 2009;

Uthicke et al., 2015; Wooldridge 2009; Wooldridge et al., 2015).

Figure 5-12: River contributions (x-axis) to the dissolved inorganic nitrogen mass to four marine NRM regions (plot head name) in 2003 (left column) and 2011 (right column). Shading groups rivers in the same NRM region: Wet Tropics - red, Burdekin - blue, Mackay-Whitsunday – yellow, Fitzroy - green. The left panel show data for the 2002-03 water year (c.a., from 1 Oct to 30 Sep), and right panel for the 2010-11 water year.

(b) Mapping annual average PN and TSS concentrations in the GBR 2003-2015 The same model developed for DIN dispersion was used to produce maps for the land-sourced PN and TSS in the GBR, except that the decay function was not included.

The model-predicted PN export to GBR lagoon is examined by its annual concentration (PN, µg/L) over 13 years (Figure 5-13 and Figure 5-14). These maps provide an estimate of how far PN can travel in GBR waters, and areas more likely to present high PN concentration.

The areas covered by model-predicted PN vary over the 13 years analysed. As observed for DIN, years with large river discharge (> 65,000,000 ML) resulted in larger areas of PN

extended across the GBR. The highest model-predicted PN concentration was observed in 2011, followed by 2008 and 2012, with maximums of 354 µg/L, 261 µg/L and 244 µg/L, respectively. The areas showing high PN concentration were relatively constant over the years, with high PN values observed in the Wet Tropics region. During years with large flows high PN areas high concentrations were also observed in the Burdekin and Mackay Whitsunday regions.

Figure 5-13: Particulate nitrogen (PN, µg/L) over the GBR lagoon 2003 water year (c.a., 1 October to 30 September). ‘Max.’ stands for the highest PN concentration. Named rivers are those with load data available and grey lines are the NRM region boundaries.

Figure 5-14: Particulate nitrogen (PN, µg/L) over the GBR lagoon 2004-2015 water year (c.a., 1 Octoberto 30 September). ‘Max.’ stands for the highest PN concentration.

Dots represent rivers with load data available and grey lines are the NRM region boundaries.

The model-predicted TSS export to the GBR lagoon was examined by its annual concentration over 13 years (

Figure 5-15) with similar patterns as observed for DIN and PN in relation to river discharge.

The highest model-predicted TSS concentration was observed in 2011, followed by 2007 and 2008, with maximums of 100 mg/L, 84 mg/L and 78 mg/L, respectively. The areas with high TSS concentration were more variable over the years compared to the DIN and PN assessments. High TSS values were observed in the Wet Tropics region over all of the years analysed, but high values were also observed in the Burdekin region in several years including 2005, 2007 and 2013, and in Mackay Whitsunday in 2010, 2011 and 2012 (Figure 5-16).

Figure 5-15: Total suspended solids (TSS, mg/L) over the GBR lagoon 2003 water year (c.a., 1 Octoberto 30 September). ‘Max.’ stands for the highest TSS concentration. Named rivers are those with load data available and grey lines are the NRM region boundaries.

Figure 5-16: Total suspended sediments (TSS, µg/L) over the GBR lagoon 2004-2015 water year (c.a., 1 Octoberto 30 September). ‘Max.’ stands for the highest TSS concentration. Dots represent rivers with load data available and grey lines are the NRM region boundaries.